Re: [PATCH RFC 1/2] powerpc/pseries: papr-vpd char driver for VPD retrieval

[Michael Ellerman <mpe@ellerman.id.au>]

Michal Suchánek <msuchanek@suse.de> writes:
> On Thu, Aug 31, 2023 at 03:34:37PM +1000, Michael Ellerman wrote:
>> Michal Suchánek <msuchanek@suse.de> writes:
>> > On Tue, Aug 22, 2023 at 04:33:39PM -0500, Nathan Lynch via B4 Relay wrote:
>> >> From: Nathan Lynch <nathanl@linux.ibm.com>
>> >> 
>> >> PowerVM LPARs may retrieve Vital Product Data (VPD) for system
>> >> components using the ibm,get-vpd RTAS function.
>> >> 
>> >> We can expose this to user space with a /dev/papr-vpd character
>> >> device, where the programming model is:
>> >> 
>> >>   struct papr_location_code plc = { .str = "", }; /* obtain all VPD */
>> >>   int devfd = open("/dev/papr-vpd", O_WRONLY);
>> >>   int vpdfd = ioctl(devfd, PAPR_VPD_CREATE_HANDLE, &plc);
>> >>   size_t size = lseek(vpdfd, 0, SEEK_END);
>> >>   char *buf = malloc(size);
>> >>   pread(devfd, buf, size, 0);
>> >> 
>> >> When a file descriptor is obtained from ioctl(PAPR_VPD_CREATE_HANDLE),
>> >> the file contains the result of a complete ibm,get-vpd sequence. The
>> >
>> > Could this be somewhat less obfuscated?
>> >
>> > What the caller wants is the result of "ibm,get-vpd", which is a
>> > well-known string identifier of the rtas call.
>> 
>> Not really. What the caller wants is *the VPD*. Currently that's done
>> by calling the RTAS "ibm,get-vpd" function, but that could change in
>> future. There's RTAS calls that have been replaced with a "version 2" in
>> the past, that could happen here too. Or the RTAS call could be replaced
>> by a hypercall (though unlikely).
>> 
>> But hopefully if the underlying mechanism changed the kernel would be
>> able to hide that detail behind this new API, and users would not need
>> to change at all.
>> 
>> > Yet this identifier is never passed in. Instead we have this new
>> > PAPR_VPD_CREATE_HANDLE. This is a completely new identifier, specific to
>> > this call only as is the /dev/papr-vpd device name, another new
>> > identifier.
>> >
>> > Maybe the interface could provide a way to specify the service name?
>> >
>> >> file contents are immutable from the POV of user space. To get a new
>> >> view of VPD, clients must create a new handle.
>> >
>> > Which is basically the same as creating a file descriptor with open().
>> 
>> Sort of. But much cleaner becuase you don't need to create a file in the
>> filesystem and tell userspace how to find it.
>
> You very much do. There is the /dev/papr-vpd and PAPR_VPD_CREATE_HANDLE
> which userspace has to know about, the PAPR_VPD_CREATE_HANDLE is not
> even possible to find at all.

Well yeah you need the device itself :)

And yes the ioctl is defined in a header, not in the filesystem, but
that's entirely normal for an ioctl based API.

cheers